324 



THE CAT. 



[CHAP. x. 



Fig. 148. FIVE DIAGRAMMATIC VERTICAL SECTIONS THROUGH THE LONG Axis OF A MATURING 

 MAMMALIAN OVUM, TO SHOW THE FORMATION OF ITS EGG-MEMBRANES. 



In Figs. 1 4 the section passes through the 

 middle of the embryo. In Fig. 5, it passes 

 a little on one side, so that the left side of 

 the embryo can be seen. 



1. Ovum in which the chorion has begun to be 



formed, with the blastoderm (and rudiment 

 of [the embryo) within it. The whole germ- 

 yesicle consists of epiblast and hypoblast, 

 in the germ area, mesoblast (m) has also 

 appeared. 



2. Ovum in which the head and tail-folds have 



contracted the umbilical aperture towards 

 the yelk-sac ; while the ascending circum- 

 vallation is seen rising at either end to form 

 the amnion. Here, then, the embryo is 

 beginning to separate from the germ- 

 vesicle (ds) . 



3. The amniotic folds being completed, have 



met in the dorsal region ; the umbilical 

 opening is more contracted, and the allan- 

 tois (at) has begun to sprout. Here we see 

 the double sac forming above the embryo, 

 the inner part of which alone becomes the 

 amnion. The intestinal canal (dd) is begin- 

 ning to be distinctly formed. 

 4. Here the inner amniotic sac or true amnion 

 is detached from the outer or false amnion, 

 which has disappeared by coalescence with 

 the inner surface of the chorion. The 

 cavity of the amnion is more distended, and 

 the yelk-sac (ds) has become smaller and 

 pedunculated. The allantois (al) projects 

 into the space between the amnion, chorion 

 and yelk-sac ; and the villi are larger, and 

 have begun to ramify. 



